Enzyme carbohydrate epimerases

Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content.28 However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested.28 29 The GDP-D-mannose-3, 5-epimerase is a well documented but poorly understood enzyme. Multistep mechanisms related to that of UDP-glucose 4-epimerase (Eqs. 20-1,15-14) can be envisioned. 

Various epimerases acting on carbohydrate derivatives and acyl-CoA derivatives were demonstrated, purified, and characterized as reviewed previously1184. Lactate race-mase (E.C. 5.1.2.1) is the first racemase to he discovered (1 41. The mechanism of lactate racemase reaction was studied with the enzyme preparations partially purified from Clostridium butyricumll8S. Hiyama et al.[1861 highly purified the enzyme from Lactobacillus sake, but little is known about its enzymological properties. In contrast, mandelate racemase (E. C. 5.1.2.2) is the enzyme best characterized among various racemases and epimerases its tertiary structure and functional groups that participate directly in catalysis has been clarified. 

Perhaps the key reaction in this chapter was the aldol condensation in Section 22.2. Although it is not mentioned in the discussion, the aldol reaction is reversible under certain circumstances. Enzymes known as aldolases can catalyze both the forward and reverse aldol reactions. An example is the retro-aldol/ aldol mechanism employed by the enzyme l-ribulose-5-phosphate-4-epimerase, found in both prokaryotes and eukaryotes, to invert the stereochemistry of the hydroxyl-bearing carbon in 150 to that in 153. In other words, 1-ribulose-5-phosphate is epimerized to give d-xylulose-5-phosphate (see Chapter 28, Section 28.1, for a discussion of these carbohydrates). 

Interesting extensions of isomerase chemistry are coupled enzyme systems. These could be advantageous for the removal of the desired isomer from the sometimes unfavourable equihbriiun to increase yields, as pointed out by Fess-ner and co-workers. Other apphcations could make rare sugars conveniently accessible from easily available starting materials, for example, in the preparation of D-psicose from o-fructose [6] by the combination of o-xylose isomerase with D-tagatose 3-epimerase. Clearly, the wide range of other available enzymes, for example, oxidoreductases, would allow for even more exciting opportunities to access unusual carbohydrates. 

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